diff --git a/pretix_public_registrations/signals.py b/pretix_public_registrations/signals.py
index 582290dc9e0c9061c1e53463602e9b197abacdee..6643841f8b1e848e30f1e2ce41301ab7ffa7a274 100644
--- a/pretix_public_registrations/signals.py
+++ b/pretix_public_registrations/signals.py
@@ -55,7 +55,6 @@ def add_public_registrations_table(sender, **kwargs):
 
 
 @receiver(signal=process_response, dispatch_uid="public_registragions_csp_headers")
-def add_public_registrations_csp_headers(sender, **kwargs):
-    response = kwargs['response']
+def add_public_registrations_csp_headers(sender, response=None, **kwargs):
     response['Content-Security-Policy'] = "img-src https://secure.gravatar.com"
     return response